Reinforced structural medium

ABSTRACT

A structural medium, useful, e.g., for constructing ships, such as icebreaker-tankers, for cold climates, having a cross-section parallel to an expected direction of impact comprising a layer of ice confined between spaced-apart, ice-impermeable structural members (such as an inner cargo-containing hull and an outer hull shell).

United States Patent Keil et al.

[111 [451 July 23, 1974 REINFORCED STRUCTURAL MEDIUM Inventors: Alfred A. H. Keil, Belmont, Mass.; Michael F. Vetter, Alexandria, Va.

Assignee: Massachusetts Institute of Technology, Cambridge, Mass.

Filed: Nov. 4, 1971 Appl. No.: 195,548

US. Cl.... 114/69, 114/74 A Int. Cl B631) 43/10 Field of Search 114/69, 74 A, 227, 50,

References Cited UNITED STATES PATENTS Leedy 114/227 1,495,310 5/1924 Stromberg 61/46 Primary Examiner-Trygve M. Blix Assistant Examiner-Stuart M. Goldstein Attorney, Agent, 0r FirmArthur A. Smith, Jr.; Robert L. Goldberg; Robert Shaw 5 7 ABSTRACT A structural medium, useful, e.g., for constructing ships, such as icebreaker-tankers, for cold climates, having a cross-section parallel to an expected direction of impact comprising a layer of ice confined between spaced-apart, ice-impermeable structural members (such as an inner cargo-containing hull and an outerhull shell).

10 Claims, 7 Drawing Figures PATENTED JUL 2 3 I914 SHEET 2 [IF 3 LOAD (POUNDS) CONCENTRATED LOAD I 1 I I MAXIMUM DEFLECTION (INCHES) 52 &

REINFORCED STRUCTURAL MEDIUM BACKGROUND OF INVENTION Thisinvention relates to structural media, and, in particular, to structural media for cold (e.g., Arctic) climates. In a particular aspect, the invention relates to constructing ships, such as icebreaker-tankers, to carry petroleum and other potential ocean pollutants in Arctic or similarly coldregions.

The great crude oil spills of the past several years have created a flurry of activity toward procedures for separating and recovering spilled oil from the ocean waters. As long as spill-prone tankers of present construction still ply the oceans, such activity will continue to be necessary. At the same time, it must be recognized that recovery procedures are merely symptondirected, and that far better than cleaning the oceans of contaminating petroleum, is to design tankers not susceptible to most spill-causing hazards. With the frequency of natural obstacles and hazards that may be expected for tankers travelling the Arctic waters, the opening of these regions to oil exploration and recovery and the presently still relatively uncontaminated and virgin conditions of theArctic waters makes urgent the design of more spill-proof icebreaker-tankers for Arctic duty.

In general, icebreaker-tankers presently designed for I Arctic travel are reinforced against impact by having thicker and thus heavierlhulls along the sides and bow, as well as heavier and more closely spaced stiffeners. Heavier hulls, however, do increase the dead weight of the ship and hence reduce its efficiency. Proposals have beenmade that such tankers be constructed with a double hull, i.e., an inner, cargo-containing hull and, spaced from it, an outer hull or skin, so that, upon impact rupturing the outer skin, the inner hull or plating would still retain the cargo. However, the space between the hulls obviously offers no impact resistance, so that, as a practical matter, most impacting hazards whichimpact with sufficient force to rupture the outer shell would in all likelihood continue on to contact with unexpectedly high resistance to applied loads and, of particular importance, to substantial impacts. In one preferred aspect, the invention features a ship, such as an icebreaker-tanker, designed to carry petroleum or other liquid cargo, having an inner water-impermeable hull constructed to confine cargo and an outer waterimpermeable hull spaced from the inner hull, at least along the sides of the ship, to define an enclosure suitable for containing ice adjacent the outer side of cargocarrying portions of the inner hull. Stiffeners, such as web frames, extending between the hulls or generally parallel to them, may be located in the ice layer as additional reinforcement.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages will be apparent to one skilled in the art from the following description of a preferred embodiment of the invention, taken together with the attached drawings thereof, in which:

F IG. 1 is a diagrammatic view, partially broken away, of a portion of an icebreaker-tanker embodying the present invention;

FIG. 2 is another diagrammatic view of one-half of the ship, of FIG. 1, taken generally along line 2-2 of FIG. 1;

FIG. 3 is an isometric view of a structural medium constructed in accordance with the present invention;

FIG.'4 is a side view of the structural medium of FIG. 3 illustrating application of test loads thereto; and,

FIGS. 5, 6, and 7 are graphs illustrating the impact and load resistance of the structural medium shown in FIGS. 3 and 4.

DESCRIPTION OF PARTICULAR EMBODIMENT FIGS. 1 and 2 show a ship 10, in particular, an icebreaker-tanker having a bow l2 specially designed for travel through ice fields. The ship has an inner steel hull and possible rupture of the inner hull, and ultimately therefore to cargo spillage. T

SUMMARY OF INVENTION The invention features a structural medium having a cross-section parallel to an expected direction of impact comprising a layer of ice confined between spaced-apart, ice-impermeable outer structural members. Although ice or ice-based mixtures (such as pycrete, a frozen woodpulp and water mixture) have been used as a structural medium (igloos, roadways, etc.), ice has generally been thought too brittle to withstand impact. Applicants have discovered, however, that an ice sandwich of moderate practical thickness ofiers 16 and an outer steel hull 18, each of conventional thickness for double hull vessels (at most as thick as Classification Societies Regulations). Ductile steels of the type currently used'to construct bows of arctic tankers which maintain ductility at sub-freezingtemperatures are preferred. The hulls are separated by a plurality of web frames 20, which define between them a plurality of wing tanks 22, which, as shown, have an optional top 24 and bottom 26 defining a plurality of enclosures 28, each of generally rectangular crosssection. The enclosures 28 are filled with ice. Bulkheads 30 (of which one is shown in FIG. 2) define cargo tanks 32. The vertical center keel 34 of the ship extends between its inner bottom 36 and outer bottom 38. The inner bottom 36 is an optional feature where, e.g., impact-resistant bottoms are desired; in such event, it might also be ice-filled.

The ice-filled enclosures 28 extend at least the conventional distances for reinforced ice belts above and below the water line 42. The dimensions of the enclosures depend, of course, on the size and type of the ship. By way of illustration, in a large tanker (say, 200,000 to 300,000 dwt) each enclosure would be about 5 feet thick between the inner and outer hulls and perhaps 40 to 50 feet long. The horizontal extent of each enclosure, i.e., the separation between the web frames 20, is determined largely by the design of the bulkheads and cargo tanks within the inner hull, it

3 being preferred that the ice-filled enclosures be disposed at least adjacent cargo-carrying regions of the inner hull.

The distance between the hulls and hence the thickness of the ice layer is largely one of convenience and economy. It is preferable, for construction and maintenance purposes, that the distance between the hulls be at least about 3" feet; a thickness of about 5 feet between steel hulls will probably prevent rupture of the inner hull except under the most extraordinary impacts, without unduly increasing the non-cargocarrying volume of the ship.

In use, this hull construction presents to a potential impacting medium, in the cross-section generally parallel to an expected direction of impact, i.e., to at least one component direction of any impact exerting a transverse or perpendicular load on the side of the ship, a three-layer ice sandwich comprising the outer hull, the ice layer, and finally the inner hull. The outer hull and then the ice layer must be ruptured by the impacting medium before rupture can extend to the inner hull and spillage occur. It has been found that not only does the ice per se present an obstacle to the impacting vehicle but it also remarkably reinforces both the outer and inner hulls, as illustrated by FIGS. 5 to 7, described in more detail below, substantially increasing the impactresistance of both hulls. Even if the impact is sufficient to crack the ice layer, nonetheless substantial strengthening of the two hulls against rupture continues. Inasmuch as icecracks generally along the lines of its component 004' to l in. grains, 3 to 5 foot layers as described-will maintain homogeneous reinforcement in all three dimensions even when substantially cracked. These cracks would, ofcourse, disappear as soon as the ice was melted and refrozen. If desired, means could be provided within the enclosures for rapidly melting and reforming the ic For forming ice from Water inthe enclosures, natural environmental Arctic waters may be relied upon, perhaps with also some thermal insulation dependent on the nature, e.g., the temperature, of the cargo, between the cargo tanks of the inner hull and the ice enclosures; or a cooling system might be provided in the enclo sures.

Where the ship is also to traverse warmer waters,-a cooling system could be incorporated within the enclosures to maintain sub-freezing temperatures, if the ice layer is desired to be maintained. However, if the absence of impacting ice hazards in the warmer water is considered a sufficient reduction in the overall probability of severe impact, the enclosures may be thawed and even drained of water and utilized instead as additional cargo compartments.

In addition to ice, reinforcing materials (which could be non-pollutant if required) such as fiberglass, sawdust, wood fibers, asbestos, shredded newspaper and the like could be mixed with water to form the ice layer and further increase the load and impact resistance of the ice sandwich.

LOAD RESISTANCE AND IMPACT RESISTANCE The load resistance and impact resistance of this structural medium were determined as follows. Four test structures 50 (FIGS. 3 and 4) were constructed, two out of 0.125 in. thick steel and two out of0. 125 in. aluminum stock. The dimensions of each structure were 2 X 16 X 20 in. (leaving room inside for a 1.750

in. thick ice layer). A plurality of parallel holes 52 were provided to fill the test structure. Although these structures or shells 50 were specifically designed as ship hulls, the general shell configuration is useful in many other structural environments where impact and loadresistance are desirable. Although in a finished construction the edges of these structures might not be fully free to rotate, nonetheless in simulated impact testing, to achieve reliable comparative results, the structures were supported on a box-shaped knife edge 54, as shown in FIG. 4, so that all of the edges of the structure were substantially free to rotate under an applied load.

A concentrated load was applied both statically and dynamically to the center of the top surface 56 of each structure 50 (corresponding to the outer hull) at the point indicated by the arrow 58 in FIG. 4, and a distributed load over the entire top surface 56 as indicated by the arrows 60. These load tests were each performed on an empty and an ice-filled steel, and an empty and an ice-filled aluminum structure. The loads were applied using a standard Youngs tensile-compression machine, with the concentrated load being applied actually over asmall annular area (o.d. 1.5 in., i.d. 0.625 in. A distributed load was applied to each structure by arranging a rubber airbag along the top surface 56 of the structure between the structure and the loadapplying crosshead of the Youngs machine. Dynamic loads were applied by dropping known weights from known heights onto the concentrated load area.

The'displacement of the bottom and (except, for distributed load application) top surface of the structure, corresponding to the outer and inner hulls, respectively, was measured by Linear Variable Differential Transformers (each having a primary and a secondary coil and a movable magnetic core producing a voltage proportional to the displacement).

FIG. 5 illustrates the improvements achieved with this ice sandwich configuration in resistance to concentrated statis loading, less than the crushing strength of the ice layer (such as a tanker might encounter from a jagged piece of ice). The curves indicate the following conditions:

5A steel, no ice, bottom surface 5B steel, ice, bottom surface 5C "steel, no ice, top surface 5D steel, ice, top surface 5E aluminum, no ice, bottom surface 5F aluminum, ice, bottom surface 5G aluminum, no ice, top surface 5H aluminum, ice, top surface This concentrated load, of course, became more of a distributed load after the ice cracked. For example, whereas without ice a load of 245 lbs. deflected the top surface of the steel-shelled structure 0.06 inches, when ice-filled a load of 2,260 lbs. was required to cause the same deflection. The force required to deflect the top surface of an aluminum-shelled structure 0.04 inch increased from 50 lbs. to over 1,500 lbs. when the aluminum structure was ice-filled. The rapid change in slope of the steel, ice-filled curves near the top was indicative of ice cracking, but the ice layer still can be seen to have substantial reinforcing strength.

FIG. 6 illustrates the improvements achieved with the ice sandwich configuration in resistance to distributed loading (such as a tanker might encounter when its hull is pinched between two ice floes the estimated total pressure on the hull, for the average 12 foot thick ice floe being about 600 psi). As previously explained, only deflections in the bottom or inner surface were measured. The curves indicate the following conditions:

6A steel, no ice, bottom surface 68 steel, ice, bottom surface 6E aluminum, no ice, bottom surface 6F aluminum, ice, bottom surface The turnaround of the unfilled aluminum model is believed due to peculiarities in the conformation and properties of its welded corners. Again, the improvement was striking. For example, whereas the bottom surface of a hollow steel-shelled structure deflected 0.02 in. under only about 3 psi., a total of over 16 psi. was required to deflect the bottom surface of an icefilled steel-shelled structure an equal amount.

FIG. 7 illustrates the improvement in impactresistance achieved with the ice layer in dynamic loading (suchas a tankermight encounter in actual travel through ice fields by colliding with large pieces of ice). Deflections in both the top and bottom surfaces of a steel-shelled structure and in the bottom surface of an aluminum-shelled structure were measured by drop ping a weight (43.25 lbs.) for distances of 0.50 to 6 inches onto the top surface of each shell. The curves indicate the following conditions:

7A steel, no ice, bottom surface 78 steel, ice, bottom surface 7C steel, no ice, top surface 7D steel, ice, top surface 7E aluminum, no ice, bottom surface 7F aluminum, ice, bottom surface Again, improvements are readily observable. For example, an impact of 173 in-lb (dropping the weight 4 in.) caused a 0.4 in. deflection in the top surface of an empty steel-shelled structure but only a 0.12 in. deflection in an ice-filled steel-shelled structure.

In addition to tanker construction, the ice-filled structural medium of this invention would be useful also in other pollutant-carrying vessels, such as barges and In g. tankers, particularly when intended for travel in the Arctic waterways. Thinner steel stock than presently required could possibly be used where icereinforced as described, or even lighter-weight structural materials such as aluminum, with the thickness of the ice layer perhaps striking anaccommodation between excessive lessening of the cargo-carrying capacity of the barge and maximization of the protection against rupturing impacts.

In addition to ship construction, this ice sandwich medium has other uses. For example, supports such as piers or pilings, which are impact-prone might, at least in Arctic regions, be provided with a hollow iceconfining shell surrounding a perhaps more fragile inner concrete or wood structure.

Other embodiments will occur to those skilled in the art and are within the following claims.

What is claimed is:

l. A ship having an inner water-impermeable hull and an outer-water impermeable hull spaced from said inner hull at least along the sides of said ship to define an enclosure between said hulls, said enclosure being filled with ice whereby the inner hull and outer hull are reinforced and the impact resistance of said ship is substantially increased.

,2. A ship according to claim 1 wherein a stiffening element issecured between said inner hull and said outer hull to maintain spacing between said hulls.

3. A ship according to claim 2 where the stiffening element extends substantially parallel to said hulls.

4. A ship according to claim 1 wherein said inner hull and said outer hull are constructed to confine cargo.

5. A ship according to claim 1 wherein said enclosure is defined between parallel surfaces of said inner and outer hulls spaced at least about 3 feet apart.

6. A ship according to claim 1 wherein said inner hull comprises tanks for containing a liquid cargo.

7. A ship according to claim 6 wherein said liquid cargo is petroleum.

8. A ship according to claim 1 in the form of an icebreaker-tanker.

9. A process for protecting a ship against rupture on impact, said ship having an inner water-impermeable hull and an outer water-impermeable hull spaced from said inner hull at least in regions located along the sides of said ship to define an enclosure, said process comprising substantially filling said enclosure with ice whereby said inner and outer hull are reinforced and said ice provides impact resistance upon impact.

10. The process of claim 9 where said inner hull and said outer hull are constructed to confine cargo and are spaced apart by at least about 3 feet. 

1. A ship having an inner water-impermeable hull and an outerwater impermeable hull spaced from said inner hull at least along the sides of said ship to define an enclosure between said hulls, said enclosure being filled with ice whereby the inner hull and outer hull are reinforced and the impact resistance of said ship is substantially increased.
 2. A ship according to claim 1 wherein a stiffening element is secured between said inner hull and said outer hull to maintain spacing between said hulls.
 3. A ship according to claim 2 where the stiffening element extends substantially parallel to said hulls.
 4. A ship according to claim 1 wherein said inner hull and said outer hull are constructed to confine cargo.
 5. A ship according to claim 1 wherein said enclosure is defined between parallel surfaces of said inner and outer hulls spaced at least about 3 feet apart.
 6. A ship according to claim 1 wherein said inner hull comprises tanks for containing a liquid cargo.
 7. A ship according to claim 6 wherein said liquid cargo is petroleum.
 8. A ship according to claim 1 in the form of an icebreaker-tanker.
 9. A process for protecting a ship against rupture on impact, said ship having an inner water-impermeable hull and an outer water-impermeable hull spaced from said inner hull at least in regions located along the sides of said ship to define an enclosure, said process comprising substantially filling said enclosure with ice whereby said inner and outer hull are reinforced and said ice provides impact resistance upon impact.
 10. The process of claim 9 where said inner hull and said outer hull are constructed to confine cargo and are spaced apart by at least about 3 feet. 